1. Field of the Invention
The present invention relates to a dielectric ceramic composition used, for example, as a dielectric layer of a multilayer ceramic capacitor and an electronic device using the dielectric ceramic composition as a dielectric layer.
2. Description of the Related Art
A multilayer ceramic capacitor as an example of electronic devices is produced by alternately stacking, for example, ceramic green sheets made by a predetermined dielectric ceramic composition and internal electrode layers having a predetermined pattern, then making the same one body to obtain a green chip, and simultaneously firing the green chip. The internal electrode layers of the multilayer ceramic capacitor are made to be one body with ceramic dielectrics by firing, so it has been necessary to select materials not reacting with ceramic dielectrics. Therefore, inevitably, platinum, palladium and other precious metals have been conventionally used as materials for composing the internal electrode layers.
While, in recent years, a dielectric ceramic composition wherein nickel and other inexpensive base metals can be used has been developed and a wide reduction of the costs has realized.
In recent years, demands for compact electronic devices have become strong as electronic circuits become higher in density, and multilayer ceramic capacitors have rapidly become more compact and gained a larger capacitance. Along therewith, a thickness per one dielectric layer in a multilayer ceramic capacitor has become thinner, so a dielectric ceramic composition capable of maintaining its reliability as a capacitor even with thin layers has been desired. Particularly, when making midvoltage capacitors used with a high rated voltage compact and high capacitance, very high reliability is required to a dielectric ceramic composition.
The present inventors have proposed a dielectric ceramic composition disclosed in the patent articles 1 and 2, etc. as techniques capable of using base metals as materials to compose internal electrodes, and by which a temperature dependence of a capacitance satisfies the X7R characteristic (xe2x88x9255 to 125xc2x0 C., xcex94C=xc2x115% or less) of the EIA Standards. All of the techniques were to improve an accelerated lifetime of insulation resistance (IR) by adding Y2O3. However, further improvement of reliability has been desired in the circumstance where capacitors rapidly become more compact and obtain a larger capacitance.
On the other hand, there is known a dielectric ceramic composition, for example, disclosed in the patent article 3 as another technique satisfying the X7R characteristic.
The above dielectric ceramic compositions are those obtained by adding an oxide of at least one kind of rare-earth elements, Sc and Y, and an oxide of at least one kind of rare-earth elements, Gd, Tb and Dy to barium titanate. Namely, the technique disclosed in the patent article 3 is to improve an accelerated lifetime of insulation resistance by satisfying the X7R characteristic of the EIA Standards by adding oxides of at least two kinds of rare-earth elements selected from each of freely divided two element groups to barium titanate.
In the technique disclosed in the patent article 3, however, since the accelerated lifetime of the insulation resistance becomes short after firing when trying to satisfy the X7R characteristic, so there was a problem of keeping balance of the X7R characteristic and the lifetime. Moreover, along with attaining a further compact size and a larger capacitance, a dielectric loss (tan xcex4) becomes large and a DC bias and other reliability are liable to decline, so improvements have been desired.
Particularly, when trying to use as a material of a midvoltage multilayer ceramic capacitor having a high rated voltage, it was necessary to thicken at least 15 xcexcm of thickness per one dielectric layer if considering the reliability.
Note that there is disclosed in the patent article 4 a dielectric material having a preferable temperature characteristic for an object of satisfying a range of the X8R characteristic of the EIA Standards. Here, a rare-earth element is added for maintaining the temperature characteristic preferable, thus, a kind of the rare-earth element is different and an ionic radius of the rare-earth element is not focused on.
The patent Article 1: The Japanese Unexamined Patent Publication No. 6-84692
The patent Article 2: The Japanese Unexamined Patent Publication No. 6-342735
The patent Article 3: The Japanese Unexamined Patent Publication No. 10-223471
The patent Article 4: The Japanese Unexamined Patent Publication No. 2000-154057
An object of the present invention is to provide a dielectric ceramic composition which has an excellent reducing resisting property, exhibits excellent temperature dependence of capacitance after firing and an improved accelerated lifetime of insulation resistance. Another object of the present invention is to provide an electronic device, such as a multilayer ceramic capacitor, having high reliability produced by using the dielectric ceramic composition, particularly to provide an electronic device, such as a midvoltage multilayer ceramic capacitor having a high rated voltage.
To attain the above objects, according to a first aspect of the present invention, there is provided a dielectric ceramic composition, comprising
a main component including barium titanate,
a fourth subcomponent including an oxide of R1 (note that R1 is at least one kind selected from a first element group composed of rare-earth elements having a effective ionic radius of less than 108 pm when having a coordination number of nine), and
a fifth subcomponent including an oxide of R2 (note that R2 is at least one kind selected from a second element group composed of rare-earth elements having a effective ionic radius of 108 pm to 113 pm when having a coordination number of nine).
Preferably, a effective ionic radius of rare-earth elements composing the first element group is over 106 pm.
Preferably, when assuming a effective ionic radius of rare-earth elements composing the first element group is r1 and a effective ionic radius of rare-earth elements composing the second element group is r2, the first element group and the second element group are composed so that a ratio of r1 and r2 (r2/r1) satisfies a relationship of 1.007 less than r2/r1 less than 1.06.
According to a second aspect of the present invention, there is provided a dielectric ceramic composition, comprising
a main component including barium titanate,
a fourth subcomponent including an oxide of R1 (note that R1 is at least one kind selected from a first element group composed of rare-earth elements having a effective ionic radius of less than 108 pm when having a coordination number of nine, and at least includes Y), and
a fifth subcomponent including an oxide of R2 (note that R2 is at least one kind selected from a second element group composed of rare-earth elements having a effective ionic radius of 108 pm to 113 pm when having a coordination number of nine).
Preferably, when assuming a effective ionic radius of Y included in the first element group is ry and a effective ionic radius of rare-earth elements composing the second element group is r2, the second element group is composed so that a ratio of ry and r2 (r2/ry) satisfies a relationship of 1.007 less than (r2/ry) less than 1.05
Preferably, when assuming a effective ionic radius of Y included in the first element group is ry and a effective ionic radius of rare-earth elements composing the second element group is r2, the second element group is composed so that a ratio of ry and r2 (r2/ry) satisfies a relationship of 1.007 less than (r2/ry) less than 1.03
According to a third aspect of the present invention, there is provided a dielectric ceramic composition, comprising
a main component including barium titanate,
a fourth subcomponent including an oxide of R1 (note that R1 is at least one kind selected from a first element group composed of rare-earth elements having a effective ionic radius of less than 108 pm when having a coordination number of nine), and
a fifth subcomponent including an oxide of R2 (note that R2 is at least one kind selected from a second element group composed of rare-earth elements having a effective ionic radius of 108 pm to 113 pm when having a coordination number of nine and at least includes Tb).
Preferably, when assuming a effective ionic radius of the rare-earth elements composing the first element group is r1 and a effective ionic radius of Tb included in the second element group is rtb, the first element group is composed so that a ratio of r1 and rtb (rtb/r1) satisfies a relationship of 1.018 less than (rtb/r1) less than 1.062.
Preferably, when assuming a effective ionic radius of the rare-earth elements composing the first element group is r1 and a effective ionic radius of Tb included in the second element group is rtb, the first element group is composed so that a ratio of r1 and rtb (rtb/r1) satisfies a relationship of 1.018 less than (rtb/r1) less than 1.022.
Preferably, a ratio of the fifth subcomponent to 100 mol of the main component is a ratio of Y or more.
Preferably, a ratio of a total of the fourth subcomponent and the fifth subcomponent to 100 mol of the main component is 10 mol or less (note that the number of mole of the fourth subcomponent and the fifth subcomponent is a ratio of R1 and R2 alone).
Preferably, ratios of the respective subcomponents to 100 mol of the main component are 0.1 to 10 mol in the fourth subcomponent (note that the number of mole of the fourth subcomponent is a ratio of R1 alone) and 0.1 to 10 mol in the fifth subcomponent (note that the number of mole of the fifth subcomponent is a ratio of R2 alone).
Preferably, a first subcomponent including at least one kind selected from MgO, CaO, SrO and BaO is further comprised, wherein a ratio of the first subcomponent to 100 mol of the main component is 0.1 to 5 mol.
Preferably, a second subcomponent including a SiO2 based sintering aid is further comprised, wherein a ratio of the second subcomponent to 100 mol of the main component is 2 to 10 mol. In this case, the sintering aid is preferably (Ba, Ca)xSiO2+x (note that x=0.8 to 1.2).
Preferably, a third subcomponent including at least one kind selected from V2O5, MoO3 and WO3 is further comprised, wherein a ratio of the third subcomponent to 100 mol of the main component is 0.5 mol or less.
Preferably, a sixth subcomponent including at least one of MnO and Cr2O3 is further comprised, wherein a ratio of the sixth subcomponent to 100 mol of the main component is 0.5 mol or less.
Preferably, a diffusion part at least including the R1 and R2 exists inside respective dielectric particles composing the dielectric ceramic composition.
Preferably, the dielectric particles comprise a ferroelectric part substantially not including the R1 and R2 and a diffusion part existing around the ferroelectric part,
a grain boundary segregation part exists around the diffusion part,
the diffusion part and grain boundary segregation part include at least the R1 and R2, and
when assuming respective existential quantities of R1 and R2 in the diffusion part are MAR1 and MAR2 and respective existential quantities of R1 and R2 in the grain boundary segregation part are MBR1 and MBR2, relationships of (MBR1/MBR2) greater than 1 and (MAR1/MAR2) less than (MBR1/MBR2).
Preferably, a value of (MAR1/MAR2) in the diffusion part gradually decreases as getting close to the ferroelectric part side from the grain boundary segregation part side.
In the dielectric ceramic composition according to the above first to third aspects, the invention according to a fourth aspect described below is preferable.
According to the fourth aspect of the present invention, there is provided a dielectric ceramic composition, comprising
a main component including barium titanate,
a first subcomponent including MgO,
a second subcomponent including a SiO2 based sintering aid,
a third subcomponent including V2O5,
a fourth subcomponent including an oxide of R1 (note that R1 is Y),
a fifth subcomponent including an oxide of R2 (note that R2 is at least one kind selected from Dy, Tb and Gd), and
a sixth subcomponent including MnO,
wherein ratios of the respective subcomponents to 100 mol of the main component are
the first subcomponent: 0.1 to 5 mol,
the second subcomponent: 2 to 10 mol
the third subcomponent: 0.5 mol or less, and
the sixth subcomponent: less than 0.25 mol.
In the fourth aspect, preferably, a ratio of a total of the fourth subcomponent and the fifth subcomponent with respect to 100 mol of the main component is 10 mol or less (note that the number of mole of the fourth subcomponent and the fifth subcomponent is a ratio of R1 and R2 alone).
In the fourth aspect, preferably, ratios of the respective subcomponents with respect to 100 mol of the main component are 0.1 to 10 mol in the fourth subcomponent (note that the number of mole of the fourth subcomponent is a ratio of R1 alone) and 0.1 to 10 mol in the fifth subcomponent (note that the number of mole of the fifth subcomponent is a ratio of R2 alone).
In the fourth subcomponent, preferably, a diffusion part including at least the R1 and R2 exists inside respective dielectric particles composing the dielectric ceramic composition.
In the fourth aspect, preferably, the dielectric particles comprise a ferroelectric part substantially not including the R1 and R2 and a diffusion part existing around the ferroelectric part,
a grain boundary segregation part exists around the diffusion part,
the diffusion part and grain boundary segregation part include at least the R1 and R2, and when assuming respective existential quantities of R1 and R2 in the diffusion part are MAR1 and MAR2 and respective existential quantities of R1 and R2 in the grain boundary segregation part are MBR1 and MBR2, relationships of (MBR1/MBR2) greater than 1 and (MAR1/MAR2) less than (MBR1/MBR2) are satisfied.
In the fourth aspect, preferably, a value of (MAR1/MAR2) in the diffusion part gradually decreases as getting close to the ferroelectric part side from the grain boundary segregation part side.
In the fourth aspect, preferably, the sintering aid is (Ba, Ca)xSiO2+x (note that x=0.8 to 1.2).
An electronic device according to the present invention is not particularly limited as far as it is an electronic device comprising a dielectric layer, and, for example, is a multilayer ceramic capacitor element comprising a capacitor element body wherein dielectric layers and internal electrode layers are alternately stacked. In the present invention, the dielectric layer is composed of any of the above dielectric ceramic compositions. A conductive material included in the internal electrode layer is not particularly limited and, for example, is Ni or a Ni alloy.
Particularly preferable electronic device is an electronic device comprising a dielectric layer composed of a dielectric ceramic composition, wherein
the dielectric ceramic composition comprises
a main component including barium titanate,
a first subcomponent including MgO,
a second subcomponent including a SiO2 based sintering aid,
a third subcomponent including V2O5,
a fourth subcomponent including an oxide of R1 (note that R1 is Y),
a fifth subcomponent including an oxide of R2 (note that R2 is at least one kind selected from Dy, Tb and Gd), and
a sixth subcomponent including MnO,
wherein ratios of the respective subcomponents to 100 mol of the main component are
the first subcomponent: 0.1 to 5 mol,
the second subcomponent: 2 to 10 mol
the third subcomponent: 0.5 mol or less,
a total of the fourth subcomponent and the fifth subcomponent: 10 mol or less (note that the number of mole of the fourth subcomponent and the fifth subcomponent is a ratio of R1 and R2 alone), and
the sixth subcomponent: less than 0.25 mol.
In this preferable electronic device, the sintering aid is preferably (Ba, Ca)xSiO2+x (note that x=0.8 to 1.2). This preferable electronic device preferably comprises a capacitor element body wherein the dielectric layers and internal electrode layers having as a main component a conductive material composed of Ni or a Ni alloy are alternately stacked.
The electronic device of the present invention is particularly fit for a midvoltage multilayer ceramic capacitor having 100V or more for rated voltage.
Note that the ionic radius in the present specification is a value based on the article xe2x80x9cR. D. Shannon, Acta Crystallogr., A32,751 (1976)xe2x80x9d.
Function and Effect of the Invention
The present inventors have studied on an effect of adding a rare-earth element to barium titanium and obtained knowledge that adding a plurality of rare-earth elements to barium titanium is effective to improve a high temperature load lifetime. Moreover, they obtained knowledge that a size of an ionic radius of the rare-earth element changes a distribution state of additive element to be added besides the rare-earth element, consequently, electric characteristic appearing on a multilayer ceramic capacitor becomes different. Then, as a result of a further study using the above knowledge as a premise, they confirmed that the larger the ionic radius of a rare-earth element to be added, the solid solubility with barium titanium particles becomes high, so the rare-earth element is distributed to a deep part of the barium titanium particles, and segregation of the rare-earth element, additive element, particularly alkaline earth element decreases, consequently, insulation resistance becomes high and reliability of a high temperature load lifetime, etc. is improved, while the specific permittivity declines and a temperature dependence of a capacitance becomes large so as not to satisfy the X7R characteristic. On the other hand, they confirmed that a temperature dependence of a capacitance becomes small when an ionic radius of a rare-earth element to be added is small, however, the rare-earth element and alkaline earth element are liable to be segregated together with Si, etc. to be added as a sintering aid agent and reliability as a capacitor is declined.
In this connection, the present inventors proceeded studies on adding a plurality of rare-earth elements having different ionic radiuses to barium titanium by focusing on an ionic radius of a rare-earth element and attained the present invention.
In the first aspect of the present invention, a rare-earth element group having a variety of a effective ionic radiuses is divided to two element groups at the boundary of 108 pm and added to barium titanium. In the second aspect, the rare-earth elements are divided to two element groups: an element group including Y and a rare-earth element group having a effective ionic radius of 108 pm or more, and added to barium titanium. In the third aspect, the rare-earth elements are divided to a rare-earth element group having a effective ionic radius of less than 108 pm and an element group including Tb and added to barium titanium.
In any of dielectric ceramic compositions according to the first to third aspects of the present invention, an excellent reducing resisting property is obtained at firing, and after firing, excellent properties are exhibited in a specific permittivity, dielectric loss, bias characteristic, breakdown voltage, temperature dependence of capacitance, etc., and an accelerated lifetime of insulation resistance is improved.
In an electronic device according to the present invention, since a dielectric layer composed of a dielectric ceramic composition of the present invention is provided, an accelerated lifetime of insulation resistance is improved, consequently, the reliability is improved. As an electronic device, while it is not particularly limited, a multilayer ceramic capacitor, piezoelectric element, chip inductor, chip varistor, chip thermistor, chip resistance and other surface-mounted (SMD) chip type electronic devices may be mentioned. Particularly, according to the fourth aspect, an electronic device suitable to midvoltage multilayer ceramic capacitors having a high rated voltage (for example 100V or more) can be provided.
Note that in the patent article 3 referred to in the Description of the Related Art above, a dielectric ceramic composition wherein a plurality of rare-earth elements selected from freely divided two groups of element groups are added to barium titanate is disclosed. However, in the publication, there is not disclosed any invention thoughts of dividing rare-earth elements to two groups based on sizes of effective ionic radiuses when having a coordination number of nine as in the present invention. Accordingly, in the publication, Y and Sc belong to one element group. The same explanation can be adopted to the patent article 4 referred to in the Description of the Related Art, as well.
Meanwhile, in the present invention, Sc is omitted. It is because Sc has a largely different ionic radius comparing with those of other rare-earth elements, so a effective ionic radius when having a coordination number of nine is not regulated. Thus, Y and Sc do not belong to one element group in the present invention.
When assuming that Sc is used as an element having a effective ionic radius of less than 108 pm, it is confirmed that the X7R characteristic can be satisfied, but an accelerated lifetime of insulation resistance has not to be improved (refer to Sample 24 in Table 3). The reason why is considered that because the ionic radius of Sc is fairly small comparing with those of other rare-earth elements, such as Y, the solid solubility with barium titanium particle is largely different from those in other rare-earth elements and an effect of suppressing segregation of alkaline-earth element cannot be obtained.